Nanowires/nanopyramids shaped light emitting diodes and photodetectors

ABSTRACT

A light emitting diode device comprising: a plurality of nanowires or nanopyramids grown on a graphitic substrate, said nanowires or nanopyramids having a p-n or p-i-n junction, a first electrode in electrical contact with said graphitic substrate; a light reflective layer in contact with the top of at least a portion of said nanowires or nanopyramids, said light reflective layer optionally acting as a second electrode; optionally a second electrode in electrical contact with the top of at least a portion of said nanowires or nanopyramids, said second electrode being essential where said light reflective layer does not act as an electrode; wherein said nanowires or nanopyramids comprise at least one group III-V compound semiconductor; and wherein in use light is emitted from said device in a direction substantially opposite to said light reflective layer.

This invention concerns the use of a thin graphitic layer as atransparent substrate for the growth of nanowires or nanopyramids whichcan be formed into LEDs and photodetectors e.g. for the emission ordetection of light in the visible or UV spectrum, in particular UV LEDsand UV photodetectors. The nanowires or nanopyramids can be providedwith a conductive and ideally reflective top contact electrode materialto enable a flip chip arrangement.

BACKGROUND

Over recent years, interest in semiconductor nanocrystals (such asnanowires and nanopyramids) has intensified as nanotechnology becomes animportant engineering discipline. Nanowires, which are also referred toas nanowhiskers, nanorods, nanopillars, nanocolumns, etc. by someauthors, have found important applications in a variety of electricaldevices such as sensors, solar cells, and light emitting diodes (LEDs).

The present invention concerns LEDs and photodetectors which inparticular emit and detect light in the ultra violet (UV) spectrum,respectively. The UV light can be classified into three separatewavelength types: UV-A: 315 to 400 nm, UV-B: 280 to 315, and UV-C: 100to 280 nm.

Applications of UV-C light (especially Deep-UV (250-280 nm) includewater and air purification, and surface disinfection by destroyingbacteria, viruses, protozoa and other microbes by directly attackingtheir DNA. Deep-UV disinfection also provides many benefits overchemical options. It cannot be overdosed, and does not produceby-products, toxins, or volatile organic compounds. Deep-UV light iswell suited to treat microorganisms which become extremely resistant tochemical disinfectants, as they are unable to develop immunity toDeep-UV radiation.

In the health sector, Deep-UV light might help sterilize medical toolsor destroy deadly virus such as H1N1, and Ebola. In food processing, UVlight can help enhance shelf life of food products. UV light emittersmight find application in consumer electronic products such as waterpurifiers, air cleaner, toothbrush sterilizer, and other sanitaryproducts.

Current UV emitters are often based on Mercury lamps which areexpensive, energy inefficient, bulky, fragile and difficult to disposeof. It would be very interesting to develop a reliable and economic LEDwhich emits in the UV area, especially in the UV-C area, which is alsothe most difficult to achieve.

The small size and low power consumption, longer operating lifetime,less maintenance, environmental friendliness and easy disposal makes UVLEDs a much more attractive solution than the likes of a Mercury lamp.

UV LEDs are usually manufactured using group III-Nitride semiconductorthin films, especially using Al rich nitride materials. The higher theAl content incorporated in the structure, the deeper wavelength of thelight can be achieved. Several research groups have demonstrated thefabrication of thin film based LEDs using AlGaN, AlInGaN and AlN.However, the maximum external quantum efficiencies (EQE) achieved so farare between 2 and 6%, and about 1% for UV-B, and UV-C LEDs, respectively[Kneissl, Semiconductor Sci and Tech. 26 (2011) 014036].

There are numerous problems in preparing a UV LED, in particular onesbased on AlGaN, AlInGaN and MN thin films, leading to very low EQEs. Itis difficult to grow high quality AlGaN thin film on conventionalsupports such as sapphire or silicon on which nanowire growth mightoccur. AlN substrates that lattice match very closely with AlGaN areexpensive to prepare and there is a lack of large size AlN wafers. Toour knowledge, the largest available wafer is 1.5″ wafer withtransparency about 60% at 265 nm.

In order to function as a UV emitter, it may also be necessary to employan electrode material that is transparent to the UV light. A commonelectrode material, indium tin oxide (ITO) is not transparent in theDeep UV region. There are other problems such as a large internalreflection at the sapphire substrate/air interface, leading to largeabsorption of the reflected Deep UV light inside the LED. Sapphire istherefore not ideal as a substrate for a UV LED. The present inventiontherefore relates to UV LEDs based on nanowires or nanopyramids asopposed to films of semiconducting material.

UV nanowire (NW) LEDs have, however, been suggested in the article Zhao,Scientific Reports 5, (2015) 8332, which discusses nitrogen polar AlGaNNWs grown on Si which are deep UV emitters. Note that the process fornanowire growth requires the growth of a GaN NW stem on the Si support.Despite of the improvement in the internal quantum efficiency (IQE) ofthese NW based LEDs compared to the thin film based LEDs, the EQE hasstill remained low due to the absorption of the emitted light by thesilicon substrate and the top contact. In addition, the nanowires grownin this study are randomly positioned leading to inhomogeneity in thecomposition and size of NWs, reducing the performance of the device.

The present inventors ideally seek UV LEDs preferably based on AlGaN,AlN or AlInGaN nanowires or nanopyramids. AlGaN or AlInGaN nanowires ornanopyramids based materials are the most suitable materials for therealization of LEDs covering the entire UV-A, UV-B, and UV-C bands.

The present inventors therefore propose a solution involving the growthof nanowires (NWs) or nanopyramids (NPs) on graphitic substrates such asgraphene. In particular, the inventors consider growingAlN/AlGaN/AlInGaN NWs or NPs on graphene. Graphene acts both as asubstrate as well as transparent and conductive contact to the NWs. Dueto the transparency of graphene across all UV wavelengths and inparticular in the UV-C wavelength region, graphene can be used as abottom contact for NW or NP based UV LED devices. Moreover, theinventors have appreciated that a preferred device design involves aflip-chip design where the bottom graphitic contact/substrate is used asthe emitting side of the LED, as that improves light extractionefficiency.

In addition, higher carrier injection efficiency is required to obtainhigher external quantum efficiency (EQE) of LEDs. However, theincreasing ionization energy of magnesium acceptors with increasing Almole fraction in AlGaN alloys makes it difficult to obtain higher holeconcentration in AlGaN alloys with higher Al content. To obtain higherhole injection efficiency (especially in the barrier layers consistingof high Al content), the inventors have devised a number of strategieswhich can be used individually or together.

The growth of nanowires on graphene is not new. In WO02012/080252, thereis a discussion of the growth of semiconductor nanowires on graphenesubstrates using molecular beam epitaxy (MBE). WO2013/104723 concernsimprovements on the '252 disclosure in which a graphene top contact isemployed on NWs grown on graphene. These previous documents are not,however, concerned with UV LED flip chips. More recently, the inventorshave described core shell nanowires grown on graphene (WO2013/190128).

US 2011/0254034 describes nanostructured LEDs emitting in the visibleregion. The device comprises a nanostructured LED with a group ofnanowires protruding from a substrate. The nanowires have a p-i-njunction and a top portion of each nanowire is covered with alight-reflective contact layer which may also act as an electrode. Whena voltage is applied between the electrode and the light-reflectivecontact layer, light is generated within the nanowire.

No one before, however, has considered an LED flip chip based onnanowires (NWs) or nanopyramids (NPs) grown on graphene.

SUMMARY OF INVENTION

Thus, viewed from one aspect, the invention provides a light emittingdiode device comprising:

a plurality of nanowires or nanopyramids grown on a graphitic substrate,said nanowires or nanopyramids having a p-n or p-i-n junction,

a first electrode in electrical contact with said graphitic substrate;

a second electrode in contact with the top of at least a portion of saidnanowires or nanopyramids optionally in the form of a light reflectivelayer;

wherein said nanowires or nanopyramids comprise at least one group III-Vcompound semiconductor. In use, light is preferably emitted in adirection substantially parallel to but opposite from the growthdirection of the nanowires.

Viewed from another aspect, the invention provides a light emittingdiode device comprising:

a plurality of nanowires or nanopyramids grown on a graphitic substrate,preferably through the holes of an optional hole-patterned mask on saidgraphitic substrate, said nanowires or nanopyramids having a p-n orp-i-n junction,

a first electrode in electrical contact with said graphitic substrate;

a light reflective layer in contact with the top of at least a portionof said nanowires or nanopyramids or in contact with a second electrodein electrical contact with the top of at least a portion of saidnanowires or nanopyramids, said light reflective layer optionally actingas the second electrode;

a second electrode in electrical contact with the top of at least aportion of said nanowires or nanopyramids, said second electrode beingessential where said light reflective layer does not act as anelectrode;

wherein said nanowires or nanopyramids comprise at least one group III-Vcompound semiconductor; and wherein in use light is emitted from saiddevice in a direction substantially opposite to said light reflectivelayer.

Viewed from another aspect, the invention provides a light emittingdiode device comprising:

a plurality of nanowires or nanopyramids grown on a graphitic substrate,preferably through the holes of an optional hole-patterned mask on saidgraphitic substrate, said nanowires or nanopyramids having a p-n orp-i-n junction,

a first electrode in electrical contact with said graphitic substrate;

a light reflective layer in contact with the top of at least a portionof said nanowires or nanopyramids, said light reflective layeroptionally acting as the second electrode;

a second electrode in electrical contact with the top of at least aportion of said nanowires or nanopyramids, said second electrode beingessential where said light reflective layer does not act as anelectrode;

wherein said nanowires or nanopyramids comprise at least one group III-Vcompound semiconductor; and wherein in use light is emitted from saiddevice in a direction substantially opposite to said light reflectivelayer.

Viewed from another aspect, the invention provides a nanostructured LEDcomprising a plurality of group III-V compound semiconductor nanowiresor nanopyramids grown epitaxially on a graphitic substrate; wherein

each of the nanowires or nanopyramids protrudes from the substrate andeach nanowire or nanopyramid comprises a p-n or p-i-n-junction;

the top part of at least a portion of said nanowires or nanopyramids iscovered with a light-reflective or transparent contact layer to form atleast one contact to group of nanowires or nanopyramids;

an electrode is in electrical contact with said graphitic substrate;

the light-reflective or transparent contact layer is in electricalcontact with the first electrode via said nanowires or nanopyramids.

Viewed from another aspect, the invention provides use of an LED deviceas hereinbefore defined as a LED, in particular in the UV region of thespectrum.

In a second embodiment, the invention relates to a photodetector. Ratherthan emitting light, the device of the invention can be adapted toabsorb light and subsequently generate a photocurrent and hence detectlight.

Thus, viewed from another aspect the invention provides a photodetectordevice comprising:

a plurality of nanowires or nanopyramids grown on a graphitic substrate,said nanowires or nanopyramids having a p-n or p-i-n junction,

a first electrode in electrical contact with said graphitic substrate;

a second electrode in contact with the top of at least a portion of saidnanowires or nanopyramids optionally in the form of a light reflectivelayer;

wherein said nanowires or nanopyramids comprise at least one group III-Vcompound semiconductor; and wherein in use light is absorbed in saiddevice.

Viewed from another aspect the invention provides a nanostructuredphotodetector comprising a plurality of group III-V compoundsemiconductor nanowires or nanopyramids grown epitaxially on a graphiticsubstrate; wherein

each of the plurality of nanowires or nanopyramids protrudes from thesubstrate and each nanowire or nanopyramid comprises a p-n- orp-i-n-junction;

a top portion of each nanowire or nanopyramid or at least one group ofnanowires or nanopyramids from the plurality of nanowires ornanopyramids is covered with a transparent contact layer to form atleast one contact to group of nanowires or nanopyramids;

an electrode is in electrical contact with said graphitic substrate;

the transparent contact layer is in electrical contact with the firstelectrode via the p-n or p-i-n-junction in said nanowires ornanopyramids.

Viewed from another aspect, the invention provides use of aphotodetector device as hereinbefore defined as a photodetector, inparticular in the UV region of the spectrum.

DEFINITIONS

By a group III-V compound semiconductor is meant one comprising at leastone element from group III and at least one element from group V. Theremay be more than one element present from each group, e.g. AlGaN (i.e. aternary compound), AlInGaN (i.e. a quaternary compound), and so on. Thedesignation Al(In)GaN implies either AlGaN or AlInGaN, i.e. that thepresence of In is optional. Any element indicated in brackets may or maynot be present.

The term nanowire is used herein to describe a solid, wire-likestructure of nanometer dimensions. Nanowires preferably have an evendiameter throughout the majority of the nanowire, e.g. at least 75% ofits length. The term nanowire is intended to cover the use of nanorods,nanopillars, nanocolumns or nanowhiskers some of which may have taperedend structures. The nanowires can be said to be in essentially inone-dimensional form with nanometer dimensions in their width ordiameter and their length typically in the range of a few 100 nm to afew μm. Ideally the nanowire diameter is between 50 and 500 nm, however,the diameter can exceed few microns (called microwires).

Ideally, the diameter at the base of the nanowire and at the top of thenanowire should remain about the same (e.g. within 20% of each other).

The term nanopyramid refers to a solid pyramidal type structure. Theterm pyramidal is used herein to define a structure with a base whosesides taper to a single point generally above the centre of the base. Itwill be appreciated that the single vertex point may appear chamferred.The nanopyramids may have multiple faces, such as 3 to 8 faces, or 4 to7 faces. Thus, the base of the nanopyramids might be a square,pentagonal, hexagonal, heptagonal, octagonal and so on. The pyramid isformed as the faces taper from the base to a central point (formingtherefore triangular faces). The triangular faces are normallyterminated with (1-101) or (1-102) planes. The triangular side surfaceswith (1-101) facets could either converge to a single point at the tipor could form a new facets ((1-102) planes) before converging at thetip. In some cases, the nanopyramids are truncated with its topterminated with {0001} planes. The base itself may comprise a portion ofeven cross-section before tapering to form a pyramidal structure begins.The thickness of the base may therefore be up to 200 nm, such as 50 nm.

The base of the nanopyramids can be 50 and 500 nm in diameter across itswidest point. The height of the nanopyramids may be 200 nm to a fewmicrons, such as 400 nm to 1 micron in length.

It will be appreciated that the substrate carries a plurality ofnanowires or nanopyramids. This may be called an array of nanowires ornanopyramids.

Graphitic layers for substrates or possibly top contacts are filmscomposed of single or multiple layers of graphene or its derivatives.The term graphene refers to a planar sheet of sp²-bonded carbon atoms ina honeycomb crystal structure. Derivatives of graphene are those withsurface modification. For example, the hydrogen atoms can be attached tothe graphene surface to form graphane. Graphene with oxygen atomsattached to the surface along with carbon and hydrogen atoms is calledas graphene oxide. The surface modification can be also possible bychemical doping or oxygen/hydrogen or nitrogen plasma treatment.

The term epitaxy comes from the Greek roots epi, meaning “above”, andtaxis, meaning “in ordered manner”. The atomic arrangement of thenanowire or nanopyramid is based on the crystallographic structure ofthe substrate. It is a term well used in this art. Epitaxial growthmeans herein the growth on the substrate of a nanowire or nanopyramidthat mimics the orientation of the substrate.

Selective area growth (SAG) is the most promising method for growingpositioned nanowires or nanopyramids. This method is different from themetal catalyst assisted vapour-liquid-solid (VLS) method, in which metalcatalyst act as nucleation sites for the growth of nanowires ornanopyramids. Other catalyst-free methods to grow nanowires ornanopyramids are self-assembly, spontaneous MBE growth, and so on, wherenanowires or nanopyramids are nucleated in random positions. Thesemethods yield huge fluctuations in the length and diameter of thenanowires or nanopyramids.

The SAG method typically requires a mask with nano-hole patterns on thesubstrate. The nanowires or nanopyramids nucleate in the holes of thepatterned mask on the substrate. This yields uniform size andpre-defined position of the nanowires or nanopyramids.

The term mask refers to the mask material that is directly deposited onthe graphitic layer. The mask material should ideally not absorb emittedlight (which could be visible, UV-A, UV-B or UV-C) in the case of an LEDor not absorb the entering light of interest in the case of aphotodetector. The mask should also be electrically non-conductive. Themask could contain one or more than one material, which include Al₂O₃,SiO₂, Si₃N₄, TiO₂, W₂O₃, and so on. Subsequently, the hole patterns inthe mask material can be prepared using electron beam lithography ornanoimprint lithography and dry or wet etching.

MBE is a method of forming depositions on crystalline substrates. TheMBE process is performed by heating a crystalline substrate in a vacuumso as to energize the substrate's lattice structure. Then, an atomic ormolecular mass beam(s) is directed onto the substrate's surface. Theterm element used above is intended to cover application of atoms,molecules or ions of that element. When the directed atoms or moleculesarrive at the substrate's surface, the directed atoms or moleculesencounter the substrate's energized lattice structure as described indetail below. Over time, the incoming atoms form a nanowire ornanopyramid.

Metal organic vapour phase epitaxy (MOVPE) also called as metal organicchemical vapour deposition (MOCVD) is an alternative method to MBE forforming depositions on crystalline substrates. In case of MOVPE, thedeposition material is supplied in the form of metal organic precursors,which on reaching the high temperature substrate decompose leaving atomson the substrate surface. In addition, this method requires a carriergas (typically H₂ and/or N₂) to transport deposition materials(atoms/molecules) across the substrate surface. These atoms reactingwith other atoms form an epitaxial layer on the substrate surface.Choosing the deposition parameters carefully results in the formation ofa nanowire or nanopyramid.

The term SPSL refers to a short period superlattice.

It will be appreciated that nanowires or nanopyramids have a p-n orp-i-n junction. The orientation of the junction does not matter (i.e.the junction can be n-i-p or n-p or p-i-n or p-n). In most cases, it ispreferred to grow n-type layer first followed by i, if used, and p-typelayers.

DETAILED DESCRIPTION OF INVENTION

This invention concerns LEDs in a flip chip arrangement or aphotodetector in a flip chip arrangement. Whilst the invention isprimarily described with reference to an LED, the reader will appreciatethat essentially the same device can be used as a photodetector. Also,whilst the invention preferably concerns the emission and detection ofUV light, the device is also applicable in other regions of theelectromagnetic spectrum, in particular the visible region.

A device according to the invention comprises a nanostructured LED witha plurality of nanowires or nanopyramids grown on a graphitic substrate.Each nanowire or nanopyramid protrudes from a substrate and thesesubstantially comprise a p-n or p-i-n junction. For completeness, it maybe that a few nanowires or nanopyramids are free of a p-n or p-i-njunction for some reason. The invention relates to devices in which theintention is that all the nanowires or nanopyramids contain thenecessary junction but encompasses devices in which a few nanowires ornanopyramids might be free of such a junction. Ideally all nanowires ornanopyramids contain the necessary junction.

A top portion of each nanowire or nanopyramid may be provided with alight-reflective layer. This may simply touch the top of the nanowiresor nanopyramids or encompass a top part of the nanowires ornanopyramids. The light-reflective layer may also act as a top contactelectrode for the device or alternatively a separate top electrode maybe provided. If an electrode is provided, a light reflective layer maybe in electrical contact with this electrode which is in electricalcontact with the top of at least a portion of said nanowires ornanopyramids. It is thus important that there is an electrode that is ingood electrical contact with both the top of the nanowires ornanopyramids top and the external circuit.

An electrode is also provided in electrical contact with the bottomportion of each nanowire or nanopyramid through the conductive graphiticsubstrate. Hence there is a circuit via the top electrode which is inelectrical contact with the other electrode via the p-n orp-i-n-junction in the nanowires or nanopyramids.

When a forward voltage is applied between the electrodes, light,preferably UV light is generated in the active region in the nanowire ornanopyramid, the device works as a LED.

When a reverse voltage is applied between the electrodes and is exposedto light, preferably UV light, the active region in the nanowire ornanopyramid absorbs the light and converts it into photocurrent, thedevice works as a photodetector.

Having a nanowire or nanopyramid grown epitaxially provides homogeneityto the formed material which may enhance various end properties, e.g.mechanical, optical or electrical properties.

Epitaxial nanowires or nanopyramids may be grown from solid, gaseous orliquid precursors. Because the substrate acts as a seed crystal, thedeposited nanowire or nanopyramid can take on a lattice structure and/ororientation similar to those of the substrate. This is different fromsome other thin-film deposition methods which deposit polycrystalline oramorphous films, even on single-crystal substrates.

Substrate for Nanowire or Nanopyramid Growth

The substrate used to grow nanowires or nanopyramids is a graphiticsubstrate, more especially it is graphene. As used herein, the termgraphene refers to a planar sheet of sp²-bonded carbon atoms that aredensely packed in a honeycomb (hexagonal) crystal lattice. Thisgraphitic substrate should preferably be no more than 20 nm inthickness. Ideally, it should contain no more than 10 layers of grapheneor its derivatives, preferably no more than 5 layers (which is called asa few-layered graphene). Especially preferably, it is a one-atom-thickplanar sheet of graphene.

The crystalline or “flake” form of graphite consists of many graphenesheets stacked together (i.e. more than 10 sheets). By graphiticsubstrate therefore, is meant one formed from one or a plurality ofgraphene sheets.

It is preferred if the substrate in general is 20 nm in thickness orless. Graphene sheets stack to form graphite with an interplanar spacingof 0.335 nm.

The graphitic substrate preferred comprises only a few such layers andmay ideally be less than 10 nm in thickness. Even more preferably, thegraphitic substrate may be 5 nm or less in thickness. The area of thesubstrate in general is not limited. This might be as much as 0.5 mm² ormore, e.g. up to 5 mm² or more such as up to 10 cm². The area of thesubstrate is thus only limited by practicalities.

In a preferred embodiment, the substrate is a laminated substrateexfoliated from a Kish graphite, a single crystal of graphite or is ahighly ordered pyrolytic graphite (HOPG). Graphene could also be grownon SiC by a sublimation method, or grown by a self-assembly method onsubstrates such as Si or Ge. Graphene can even be grown by MBE directlyon such substrates.

Alternatively, the substrate could be grown on a Ni film or Cu foil byusing a chemical vapour deposition (CVD) method. The substrate could bea CVD-grown graphene substrate on metallic films or foils made of e.g.Cu, Ni, or Pt.

These CVD-grown graphitic layers can be chemically exfoliated from themetal foil such as a Ni or Cu film by etching or by an electrochemicaldelamination method. The graphitic layers after exfoliation are thentransferred and deposited to the supporting carrier for nanowire ornanopyramid growth. During the exfoliation and transfer, e-beam resistor photoresist may be used to support the thin graphene layers. Thesesupporting materials can be easily removed by acetone after deposition.

Whilst it is preferred if the graphitic substrate is used withoutmodification, the surface of the graphitic substrate can be modified.For example, it can be treated with plasma of hydrogen, oxygen,nitrogen, NO₂ or their combinations. Oxidation of the substrate mightenhance nanowire or nanopyramid nucleation. It may also be preferable topretreat the substrate, for example, to ensure purity before nanowire ornanopyramid growth. Treatment with a strong acid such as HF or BOE is anoption. Substrates might be washed with iso-propanol, acetone, orn-methyl-2-pyrrolidone to eliminate surface impurities.

The cleaned graphitic surface can be further modified by doping. Dopantatoms or molecules may act as a seed for growing nanowires ornanopyramids. A solution of FeCl₃, AuCl₃ or GaCl₃ could be used in adoping step.

The graphitic layers, more preferably graphene, are well known for theirsuperior optical, electrical, thermal and mechanical properties. Theyare very thin but very strong, light, flexible, and impermeable. Mostimportantly in the present invention they are highly electrically andthermally conducting, and transparent. Compared to other transparentconductors such as ITO, ZnO/Ag/ZnO, Al doped ZnO and TiO₂/Ag/TiO₂ whichare commercially used now, graphene has been proven to be much moretransparent (˜98% transmittance in the UV spectral range of interestfrom 200 to 400 nm in wavelength) and conducting (<1000 Ohm□⁻¹ sheetresistance for 1 nm thickness).

Support for Substrate

The graphitic substrate may need to be supported in order to allowgrowth of the nanowires or nanopyramids thereon. The substrate can besupported on any kind of material including conventional semiconductorsubstrates and transparent glasses. It is preferred if the support istransparent so that the substrate does not block light from exiting orentering the device.

Examples of preferred substrates include fused silica, fused quartz,fused alumina, silicon carbide or AlN. The use of fused silica or quartzis preferred, especially fused silica. The support should be inert.

The thickness of the support is not important as long as it acts tosupport the substrate and is transparent. The term transparent is usedhere to mean that the support allows transmission of light, inparticular UV light. In particular, it is preferred if the support istransparent to UV-B and UV-C light.

In theory, once the nanowires or nanopyramids are grown, the supportmight be removed (e.g. by etching) or the nanowires or nanopyramids canbe peeled away from the support. If the support is removed orpotentially replaced by another support structure, that might allow theuse of supports that are not transparent during the nanowire ornanopyramid growth process. The use therefore of a LED in the absence ofa support is within the scope of the invention. It is however preferredif a support is present in the LED device.

Intermediate Layer

The graphitic substrate is provided in a sheet and can potentially havea higher than desired sheet resistance. Sheet resistance is a measure oflateral resistance of a thin film that is nominally uniform inthickness. In order to reduce sheet resistance, it is preferred if anintermediate layer is provided between the graphitic substrate and thesupport. That intermediate layer is preferably hexagonal boron nitride(hBN) or could be a silver nanowire network or a metallic grid. Theintermediate layer may be present before nanowire or nanopyramid growth.

In an alternative embodiment, the intermediate layer can be appliedafter the support has been removed. Hence NWs can be grown on thegraphene layer carried on a support, the support then removed and theintermediate layer then applied on the back side of the graphenesubstrate (i.e. opposite the grown nanowires or nanopyramids).

The presence of this intermediate layer reduces the sheet resistance ofthe graphitic substrate and therefore enhances the performance of thedevice. In fact, the use of silver nanowires as an intermediate layerhas been found to reduce the sheet resistance of the graphene to as lowas 16 ohms□⁻¹.

A further option for reducing sheet resistance is to employ two or moreseparate graphitic layers. Whilst therefore the nanowires ornanopyramids are grown on surface of a graphitic substrate, the devicecan be provided with further graphitic layers on the opposite side tothe nanowire or nanopyramid carrying surface.

Again, it will be important that the intermediate layer is transparentto light, in particular UV light and especially UV-B and UV-C.

The thickness of the intermediate layer is not critical but as it actsto reduce sheet resistance, it is ideally as thin as possible, in orderto carry out its desired function and in the case of hBN, may be acouple of monolayers. It may therefore be around the same thickness asthe substrate layer. Suitable thicknesses are therefore 10 to 200 nm,such as 20 to 100 nm.

Growth of Nanowires or Nanopyramids

In order to prepare nanowires or nanopyramids of commercial importance,it is preferred that these grow epitaxially on the substrate. It is alsoideal if growth occurs perpendicular to the substrate and ideallytherefore in the [0001] (for hexagonal crystal structure) direction.

The present inventors have determined that epitaxial growth on graphiticsubstrates is possible by determining a possible lattice match betweenthe atoms in the semiconductor nanowire or nanopyramid and the carbonatoms in the graphene sheet.

The carbon-carbon bond length in graphene layers is about 0.142 nm.Graphite has hexagonal crystal geometry. The present inventors havepreviously realised that graphite can provide a substrate on whichsemiconductor nanowires or nanopyramids can be grown as the latticemismatch between the growing nanowire or nanopyramid material and thegraphitic substrate can be very low.

The inventors have realised that due to the hexagonal symmetry of thegraphitic substrate and the hexagonal symmetry of the semiconductoratoms in the (0001) planes of a nanowire or nanopyramid growing in the[0001] direction with a hexagonal crystal structure), a lattice matchcan be achieved between the growing nanowires or nanopyramids and thesubstrate. A comprehensive explanation of the science here can be foundin WO2013/104723.

Without wishing to be limited by theory, due to the hexagonal symmetryof the carbon atoms in graphitic layers, and the hexagonal symmetry ofthe atoms in the (111) planes of a nanowire or nanopyramid growing in[111] direction with a cubic crystal structure (or in the (0001) planesof a nanowire or nanopyramid growing in the [0001] crystal directionwith a hexagonal crystal structure), a close lattice match between thegraphitic substrate and semiconductor can be achieved when thesemiconductor atoms are placed above the carbon atoms of the graphiticsubstrate, ideally in a hexagonal pattern. This is a new and surprisingfinding and can enable the epitaxial growth of nanowires or nanopyramidson graphitic substrates.

The different hexagonal arrangements of the semiconductor atoms asdescribed in WO02013/104723, can enable semiconductor nanowires ornanopyramids of such materials to be vertically grown to formfree-standing nanowires or nanopyramids on top of a thin carbon-basedgraphitic material.

In a growing nanopyramid, the triangular faces are normally terminatedwith (1-101) or (1-102) planes. The triangular side surfaces with(1-101) facets could either converge to a single point at the tip orcould form a new facets ((1-102) planes) before converging at the tip.In some cases, the nanopyramids are truncated with its top terminatedwith {0001} planes.

Whilst it is ideal that there is no lattice mismatch between a growingnanowire or nanopyramid and the substrate, nanowires or nanopyramids canaccommodate much more lattice mismatch than thin films for example. Thenanowires or nanopyramids of the invention may have a lattice mismatchof up to about 10% with the substrate and epitaxial growth is stillpossible. Ideally, lattice mismatches should be 7.5% or less, e.g. 5% orless.

For some semiconductors like hexagonal GaN (a=3.189 Å), hexagonal AlN(a=3.111 Å), the lattice mismatch is so small (<˜5%) that excellentgrowth of these semiconductor nanowires or nanopyramids can be expected.

Growth of nanowires/nanopyramids can be controlled through flux ratios.Nanopyramids are encouraged, for example if high group V flux isemployed.

The nanowires grown in the present invention may be from 250 nm toseveral microns in length, e.g. up to 5 microns. Preferably thenanowires are at least 1 micron in length. Where a plurality ofnanowires are grown, it is preferred if they all meet these dimensionrequirements. Ideally, at least 90% of the nanowires grown on asubstrate will be at least 1 micron in length. Preferably substantiallyall the nanowires will be at least 1 micron in length.

Nanopyramids may be 250 nm to 1 micron in height, such as 400 to 800 nmin height, such as about 500 nm.

Moreover, it will be preferred if the nanowires or nanopyramids grownhave the same dimensions, e.g. to within 10% of each other. Thus, atleast 90% (preferably substantially all) of the nanowires ornanopyramids on a substrate will preferably be of the same diameterand/or the same length (i.e. to within 10% of the diameter/length ofeach other). Essentially, therefore the skilled man is looking forhomogeneity and nanowires or nanopyramids that are substantially thesame in terms of dimensions.

The length of the nanowires or nanopyramids is often controlled by thelength of time for which the growing process runs. A longer processtypically leads to a (much) longer nanowire or nanopyramid.

The nanowires have typically a hexagonal cross sectional shape. Thenanowire may have a cross sectional diameter of 25 nm to several hundrednm (i.e. its thickness). As noted above, the diameter is ideallyconstant throughout the majority of the nanowire. Nanowire diameter canbe controlled by the manipulation of the ratio of the atoms used to makethe nanowire as described further below.

Moreover, the length and diameter of the nanowires or nanopyramids canbe affected by the temperature at which they are formed. Highertemperatures encourage high aspect ratios (i.e. longer and/or thinnernanowires or nanopyramids). The diameter can also be controlled bymanipulating the nanohole opening size of the mask layer. The skilledman is able to manipulate the growing process to design nanowires ornanopyramids of desired dimensions.

The nanowires or nanopyramids of the invention are formed from at leastone III-V compound semi-conductor. Preferably, the nanowires ornanopyramids consists of group III-V compounds only optionally doped asdiscussed below. Note that there may be more than one different groupIII-V compound present but it is preferred if all compounds present aregroup III-V compounds.

Group III element options are B, Al, Ga, In, and Tl. Preferred optionshere are Ga, Al and In.

Group V options are N, P, As, Sb. All are preferred, especially N.

It is of course possible to use more than one element from group IIIand/or more than one element from group V. Preferred compounds fornanowire or nanopyramid manufacture include AlAs, GaSb, GaP, GaN, AlN,AlGaN, AlGaInN, GaAs, InP, InN, InGaAs, InSb, InAs, or AlGaAs. Compoundsbased on Al, Ga and In in combination with N are most preferred. The useof GaN, AlGaN, AlInGaN or AlN is highly preferred.

It is most preferred if the nanowires or nanopyramids consist of Ga, Al,In and N (along with any doping atoms as discussed below).

Whilst the use of binary materials is possible, the use of ternarynanowires or nanopyramids in which there are two group III cations witha group V anion are preferred here, such as AlGaN. The ternary compoundsmay therefore be of formula XYZ wherein X is a group III element, Y is agroup III different from X, and Z is a group V element. The X to Y molarratio in XYZ is preferably 0.1 to 0.9, i.e. the formula is preferablyX_(x)Y_(1-x)Z where subscript x is 0.1 to 0.9.

Quaternary systems might also be used and may be represented by theformula A_(x)B_(1-x)C_(y)D_(1-y) where A, B and C are different groupIII elements and D is a group V element. Again subscripts x and y aretypically 0.1 to 0.9. Other options will be clear to the skilled man.

The growth of AlGaN and AlInGaN nanowires or nanopyramids is especiallypreferred. The wavelength of light emitted by a device containing thesenanowires or nanopyramids can be tailored by manipulating the content ofAl, In and Ga. Alternatively, the pitch and/or diameter of the nanowiresor nanopyramids can be varied to change the nature of the light emitted.

It is further preferred if the nanowires or nanopyramids contain regionsof differing compounds. The nanowire or nanopyramid might thereforecontain a region of a first group III-V semiconductor such as GaNfollowed by a region of a different III-V semi-conductor such as AlGaN.Nanowires or nanopyramids can contain multiple regions such as two ormore or three or more. These regions might be layers in an axially grownnanowire or shells in a radially grown nanowire or nanopyramid.

Doping

The nanowires or nanopyramids of the invention need to contain a p-n orp-i-n junction. Devices of the invention, especially those based on ap-i-n junction are therefore optionally provided with an undopedintrinsic semiconductor region between a p-type semiconductor and ann-type semiconductor region. The p-type and n-type regions are typicallyheavily doped because they are used for ohmic contacts.

It is therefore preferred that the nanowires or nanopyramids are doped.Doping typically involves the introduction of impurity ions into thenanowire or nanopyramid, e.g. during MBE or MOVPE growth. The dopinglevel can be controlled from ˜10¹⁵/cm³ to 10²⁰/cm³. The nanowires ornanopyramids can be p-type doped or n-type doped as desired. Dopedsemiconductors are extrinsic conductors.

The n(p)-type semiconductors have a larger electron (hole) concentrationthan hole (electron) concentration by doping an intrinsic semiconductorwith donor (acceptor) impurities. Suitable donor (acceptors) for III-Vcompounds, especially nitrides, can be Si (Mg, Be and Zn). Dopants canbe introduced during the growth process or by ion implantation of thenanowires or nanopyramids after their formation.

As previously noted, higher carrier injection efficiency is required toobtain higher external quantum efficiency (EQE) of LEDs. However, theincreasing ionization energy of Mg acceptors with increasing Al contentin AlGaN alloys makes it difficult to obtain higher hole concentrationin AlGaN alloys with higher Al content. To obtain higher hole injectionefficiency (especially in the barrier layers consisting of high Alcontent), the inventors have devised a number of strategies which can beused individually or together.

There are problems to overcome in the doping process therefore. It ispreferred if the nanowires or nanopyramids of the invention comprise Al.The use of Al is advantageous as high Al content leads to high bandgaps, enabling UV-C LED emission from the active layer(s) of nanowiresor nanopyramids and/or avoiding absorption of the emitted light in thedoped barrier layers. Where the band gap is high, it is less likely thatUV light is absorbed by this part of the nanowires or nanopyramids. Theuse therefore of AlN or AlGaN in nanowires or nanopyramids is preferred.

However, p-type doping of AlGaN or AlN to achieve high electricalconductivity (high hole concentration) is challenging as the ionizationenergy of Mg or Be acceptors increases with increasing Al content inAlGaN alloys. The present inventors propose various solutions tomaximise electrical conductivity (i.e. maximise hole concentration) inAlGaN alloys with higher average Al content.

Where the nanowires or nanopyramids comprise AlN or AlGaN, achievinghigh electrical conductivity by introducing p-type dopants is achallenge. One solution relies on a short period superlattice (SPSL). Inthis method, we grow a superlattice structure consisting of alternatinglayers with different Al content instead of a homogeneous AlGaN layerwith higher Al composition. For example, the barrier layer with 35% Alcontent could be replaced with a 1.8 to 2.0 nm thick SPSL consisting of,for example, alternating Al_(x)Ga_(1-x)N:Mg/Al_(y)Ga_(1-y)N:Mg withx=0.30/y=0.40. The low ionization energy of acceptors in layers withlower Al composition leads to improved hole injection efficiency withoutcompromising on the barrier height in the barrier layer. This effect isadditionally enhanced by the polarization fields at the interfaces. TheSPSL can be followed with a highly p-doped GaN:Mg layer for better holeinjection.

More generally, the inventors propose to introduce a p-type dopedAl_(x)Ga_(1-x)N/Al_(y)Ga_(1-y)N short period superlattice (i.e.alternating thin layers of Al_(x)Ga_(1-x)N and Al_(y)Ga_(1-y)N) into thenanowires or nanopyramid structure, where the Al mole fraction x is lessthan y, instead of a p-type doped Al_(z)Ga_(1-z)N alloy where x<z<y. Itis appreciated that x could be as low as 0 (i.e. GaN) and y could be ashigh as 1 (i.e. AlN). The superlattice period should preferably be 5 nmor less, such as 2 nm, in which case the superlattice will act as asingle Al_(z)Ga_(1-z)N alloy (with z being a layer thickness weightedaverage of x and y) but with a higher electrical conductivity than thatof the Al_(z)Ga_(1-z)N alloy, due to the higher p-type doping efficiencyfor the lower Al content Al_(x)Ga_(1-x)N layers.

In the nanowires or nanopyramids comprising a p-type doped superlattice,it is preferred if the p-type dopant is an alkali earth metal such as Mgor Be.

A further option to solve the problem of doping an Al containingnanowire/nanopyramid follows similar principles. Instead of asuperlattice containing thin AlGaN layers with low or no Al content, ananostructure can be designed containing a gradient of Al content (molefraction) in the growth direction of the AlGaN within the nanowires ornanopyramids. Thus, as the nanowires or nanopyramids grow, the Alcontent is reduced/increased and then increased/reduced again to createan Al content gradient within the nanowires or nanopyramids.

This may be called polarization doping. In one method, the layers aregraded either from GaN to AlN or AlN to GaN. The graded region from GaNto AlN and AlN to GaN may lead to n-type and p-type conduction,respectively. This can happen due to the presence of dipoles withdifferent magnitude compared to its neighbouring dipoles. The GaN to AlNand AlN to GaN graded regions can be additionally doped with n-typedopant and p-type dopant respectively.

In a preferred embodiment, p-type doping is used in AlGaN nanowiresusing Be as a dopant.

Thus, one option would be to start with a GaN nanowire/nanopyramid andincrease Al and decrease Ga content gradually to form AlN, perhaps overa growth thickness of 100 nm. This graded region could act as a p- orn-type region, depending on the crystal plane, polarity and whether theAl content is decreasing or increasing in the graded region,respectively. Then the opposite process is effected to produce GaN oncemore to create an n- or p-type region (opposite to that previouslyprepared). These graded regions could be additionally doped with n-typedopants such as Si and p-type dopants such as Mg or Be to obtain n- orp-type regions with high charge carrier density, respectively. Thecrystal planes and polarity is governed by the type ofnanowire/nanopyramid as is known in the art.

Viewed from another aspect therefore, the nanowires or nanopyramids ofthe invention comprise Al, Ga and N atoms wherein during the growth ofthe nanowires or nanopyramids the concentration of Al is varied tocreate an Al concentration gradient within the nanowires ornanopyramids.

In a third embodiment, the problem of doping in an Al containingnanowire or nanopyramid is addressed using a tunnel junction. A tunneljunction is a barrier, such as a thin layer, between two electricallyconducting materials. In the context of the present invention, thebarrier functions as an ohmic electrical contact in the middle of asemiconductor device.

In one method, a thin electron blocking layer is inserted immediatelyafter the active region, which is followed by a p-type doped AlGaNbarrier layer with Al content higher than the Al content used in theactive layers. The p-type doped barrier layer is followed by a highlyp-type doped barrier layer and a very thin tunnel junction layerfollowed by an n-type doped AlGaN layer. The tunnel junction layer ischosen such that the electrons tunnel from the valence band in p-AlGaNto the conduction band in the n-AlGaN, creating holes that are injectedinto the p-AlGaN layer.

More generally, it is preferred if the nanowire or nanopyramid comprisestwo regions of doped GaN (one p- and one n-doped region) separated by anAl layer, such as a very thin Al layer. The Al layer might be a few nmthick such as 1 to 10 nm in thickness. It is appreciated that there areother optional materials that can serve as a tunnel junction whichincludes highly doped InGaN layers.

It is particularly surprising that doped GaN layers can be grown on theAl layer.

In one embodiment therefore, the invention provides a nanowire ornanopyramid having a p-type doped (Al)GaN region and an n-type doped(Al)GaN region separated by an Al layer.

The nanowires or nanopyramids of the invention can be grown to have aheterostructured form radially or axially. For example for an axialheterostructured nanowire or nanopyramid, p-n junction can be axiallyformed by growing a p-type doped core first, and then continue with ann-doped core (or vice versa). An intrinsic region can be positionedbetween doped cores for a p-i-n nanowire or nanopyramid.

For a radially heterostructured nanowire or nanopyramid, p-n junctioncan be radially formed by growing the p-doped nanowire or nanopyramidcore first, and then the n-doped semiconducting shell is grown (or viceversa). An intrinsic shell can be positioned between doped regions for ap-i-n nanowire or nanopyramid.

It is preferred if the nanowires are grown axially and are thereforeformed from a first section and a second section axially up the nanowireor nanopyramid. The two sections are doped differently to generate a p-njunction or p-i-n junction. The top or bottom section of the nanowire isthe p-doped or n-doped section.

In a p-i-n nanowire or nanopyramid, when charge carriers are injectedinto the respective p- and n-regions, they recombine in the i-region,and this recombination generates light. In a p-n junction case,recombination will occur in the space charge region (as there is nointrinsic region). The light is generated inside each nanowire ornanopyramid randomly and emitted in all directions. One problem withsuch a structure is that a substantial fraction of the generated lightis wasted, as only a portion is directed in a desired direction. The usetherefore of a reflective layer ensures that the emitted light isdirected out from the device in a desired direction, in particularopposite to the reflective layer. In particular, light is reflected outthrough the substrate and support layers (these being opposite to thelight reflective layer).

In the photodetector embodiment, the reflective layer is not essentialbut if present, may reflect back light on to the nanowires ornanopyramids for detection that would otherwise be lost.

The nanowires or nanopyramids of the invention preferably growepitaxially. They attach to the underlying substrate through covalent,ionic or quasi van der Waals binding. Accordingly, at the junction ofthe substrate and the base of the nanowire or nanopyramid, crystalplanes are formed epitaxially within the nanowire or nanopyramid. Thesebuild up, one upon another, in the same crystallographic direction thusallowing the epitaxial growth of the nanowire or nanopyramid. Preferablythe nanowires or nanopyramids grow vertically. The term vertically hereis used to imply that the nanowires or nanopyramids grow perpendicularto the substrate. It will be appreciated that in experimental sciencethe growth angle may not be exactly 90° but the term vertically impliesthat the nanowires or nanopyramids are within about 10° ofvertical/perpendicular, e.g. within 5°. Because of the epitaxial growthvia covalent, ionic or quasi van der Waals bonding, it is expected thatthere will be an intimate contact between the nanowires or nanopyramidsand the graphitic substrate. To enhance the contact property further,the graphitic substrate can be doped to match the major carriers ofgrown nanowires or nanopyramids.

Because nanowires or nanopyramids are epitaxially grown involvingphysical and chemical bonding to substrates at high temperature, thebottom contact is preferably ohmic.

It will be appreciated that the substrate comprises a plurality ofnanowires or nanopyramids. Preferably the nanowires or nanopyramids growabout parallel to each other. It is preferred therefore if at least 90%,e.g. at least 95%, preferably substantially all nanowires ornanopyramids grow in the same direction from the same plane of thesubstrate.

It will be appreciated that there are many planes within a substratefrom which epitaxial growth could occur. It is preferred ifsubstantially all nanowires or nanopyramids grow from the same plane. Itis preferred if that plane is parallel to the substrate surface. Ideallythe grown nanowires or nanopyramids are substantially parallel.Preferably, the nanowires or nanopyramids grow substantiallyperpendicular to the substrate.

The nanowires or nanopyramids of the invention should preferably grow inthe [0001] direction for nanowires or nanopyramids with hexagonalcrystal structure. If the nanowire has a hexagonal crystal structure,then the (0001) interface between the nanowire and the graphiticsubstrate represents the plane from which axial growth takes place. Thenanowires or nanopyramids are preferably grown by MBE or MOVPE. In theMBE method, the substrate is provided with a molecular beam of eachreactant, e.g. a group III element and a group V element preferablysupplied simultaneously. A higher degree of control of the nucleationand growth of the nanowires or nanopyramids on the graphitic substratemight be achieved with the MBE technique by using migration-enhancedepitaxy (MEE) or atomic-layer MBE (ALMBE) where e.g. the group III and Velements can be supplied alternatively.

A preferred technique in case of nitrides is plasma assistedsolid-source MBE, in which very pure elements such as gallium,aluminium, and indium are heated in separate effusion cells, until theybegin to slowly evaporate. The rf-plasma nitrogen source is typicallyused to produce low energy beams of nitrogen atoms. The gaseous elementsthen condense on the substrate, where they may react with each other. Inthe example of gallium and nitrogen, single-crystal GaN is formed. Theuse of the term “beam” implies that evaporated atoms (e.g. gallium) andnitrogen atoms from the plasma source do not interact with each other orvacuum chamber gases until they reach the substrate.

MBE takes place in ultra high vacuum, with a background pressure oftypically around 10⁻¹⁰ to 10⁻⁹ Torr. Nanostructures are typically grownslowly, such as at a speed of up to a few μm per hour. This allowsnanowires or nanopyramids to grow epitaxially and maximises structuralperformance.

The nature of the light emitted is a function of the diameter andcomposition of the nanowire or nanopyramid. In order to tune the bandgap of the nanowire or nanopyramid temperature and fluxes can be used.(Nanotechnology 25 (2014) 455201).

In the MOVPE method, the substrate is kept in a reactor in which thesubstrate is provided with a carrier gas and a metal organic gas of eachreactant, e.g. a metal organic precursor containing a group III elementand a metal organic precursor containing a group V element. The typicalcarrier gases are hydrogen, nitrogen, or a mixture of the two. A higherdegree of control of the nucleation and growth of the nanowires ornanopyramids on the graphitic substrate might be achieved with the MOVPEtechnique by using pulsed layer growth technique, where e.g. the groupIII and V elements can be supplied alternatively.

Selective Area Growth of Nanowires or Nanopyramids

The nanowires or nanopyramids of the invention are preferably grown byselective area growth (SAG) method. This method may require a mask withnano-hole patterns deposited on the graphitic layers.

In order to prepare a more regular array of nanowires or nanopyramidswith better homogeneity in height and diameter of grown nanowires ornanopyramids, the inventors envisage the use of a mask on the substrate.This mask can be provided with regular holes, where nanowires ornanopyramids can grow homogeneously in size in a regular array acrossthe substrate. The hole patterns in the mask can be easily fabricatedusing conventional photo/e-beam lithography or nanoimprinting. Focusedion beam technology may also be used in order to create a regular arrayof nucleation sites on the graphitic surface for the nanowire ornanopyramid growth.

Thus a mask can be applied to the substrate and etched with holesexposing the substrate surface, optionally in a regular pattern.Moreover, the size and the pitch of the holes can be carefullycontrolled. By arranging the holes regularly, a regular pattern ofnanowires or nanopyramids can be grown.

Moreover, the size of the holes can be controlled to ensure that onlyone nanowire or nanopyramid can grow in each hole. Finally, the holescan be made of a size where the hole is sufficiently large to allownanowire or nanopyramid growth. In this way, a regular array ofnanowires or nanopyramids can be grown.

By varying the size of the holes, one could control the size of thenanowire or nanopyramid. By varying the pitch of the holes, one couldoptimize the light extraction of light from the nanowires ornanopyramids.

The mask material can be any material which does not damage theunderlying substrate when deposited. The mask should also be transparentto the emitted light (LED) and entering light (photodetector). Theminimum hole size might be 50 nm, preferably at least 100-200 nm. Thethickness of the mask can be 10 to 100 nm, such as 10 to 40 nm.

The mask itself can be made of an inert compound, such as silicondioxide or silicon nitride. In particular, the hole-patterned maskcomprises at least one insulating material such as SiO₂, Si₃N₄, HfO₂,TiO₂ or Al₂O₃ e.g. deposited by e-beam evaporation, CVD, PE-CVD,sputtering, or ALD. The mask can therefore be provided on the substratesurface by any convenient technique such as by electron beam deposition,CVD, plasma enhanced-CVD, sputtering, and atomic layer deposition (ALD).

The use of a Ti mask that is either nitridated/oxidized before thenanowire growth, is particularly preferred as such a mask has been foundto allow growth of uniform NWs (e.g. see J. Crystal Growth 311(2009)2063-68).

The selective area growth method yields nanowires or nanopyramids ofuniform length, and diameter at predefined positions. The nanowires ornanopyramids can also be grown without mask with nano-hole patterns. Insuch case, the nanowires or nanopyramids will have non-uniform sizes(length and diameter), and located at random positions. These methodsare different from catalyst-assisted growth methods used for the growthof other type of III-V nanowires or nanopyramids such as GaAs.

In one embodiment, it is preferred if no mask is used to grow thenanowires or nanopyramids of the invention. Moreover, the presentinventors have found that nanowire density can be maximised in theabsence of a mask. Nanowire densities of at least 20 nanowires persquare micrometer are possible, such as at least 25 nanowires per squaremicrometer. These very high nanowire densities are particularlyassociated with GaN or AlGaN nanowires.

For the nanowire or nanopyramid growth, the graphitic substratetemperature can then be set to a temperature suitable for the growth ofthe nanowire or nanopyramid in question. The growth temperature may bein the range 300 to 1000° C. The temperature employed is, however,specific to the nature of the material in the nanowire or nanopyramidand the growth method. For GaN grown by MBE, a preferred temperature is700 to 950° C., e.g. 750 to 900° C., such as 760° C. For AlGaN the rangeis slightly higher, for example 780 to 980° C., such as 830 to 950° C.,e.g. 840° C.

It will be appreciated therefore that the nanowires or nanopyramids cancomprise different group III-V semiconductors within the nanowire ornanopyramid, e.g. starting with a GaN stem followed by an AlGaNcomponent or AlGaInN component and so on.

Nanowire or nanopyramid growth in MBE can be initiated by opening theshutter of the Ga effusion cell, the nitrogen plasma cell, and thedopant cell simultaneously initiating the growth of doped GaN nanowiresor nanopyramids, hereby called as stem. The length of the GaN stem canbe kept between 10 nm to several 100s of nanometers. Subsequently, onecould increase the substrate temperature if needed, and open the Alshutter to initiate the growth of AlGaN nanowires or nanopyramids. Onecould initiate the growth of AlGaN nanowires or nanopyramids ongraphitic layers without the growth of GaN stem. n- and p-type dopednanowires or nanopyramids can be obtained by opening the shutter of then-type dopant cell and p-type dopant cell, respectively, during thenanowire or nanopyramid growth. For example, Si dopant cell for n-typedoping of nanowires or nanopyramids, and Mg dopant cell for p-typedoping of nanowires or nanopyramids.

The temperature of the effusion cells can be used to control growthrate. Convenient growth rates, as measured during conventional planar(layer by layer) growth, are 0.05 to 2 μm per hour, e.g. 0.1 μm perhour. The ratio of Al/Ga can be varied by changing the temperature ofthe effusion cells.

The pressure of the molecular beams can also be adjusted depending onthe nature of the nanowire or nanopyramid being grown. Suitable levelsfor beam equivalent pressures are between 1×10⁻⁷ and 1×10⁻⁴ Torr.

The beam flux ratio between reactants (e.g. group III atoms and group Vmolecules) can be varied, the preferred flux ratio being dependent onother growth parameters and on the nature of the nanowire or nanopyramidbeing grown. In the case of nitrides, nanowires or nanopyramids arealways grown under nitrogen rich conditions.

The nanowires or nanopyramids of the invention preferably comprise n-por n-i-p Al(In)GaN or AlGaN nanowires or nanopyramids. The active layer(i-region) could consist of Al_(x1)Ga_(y1)N/Al_(x2)Ga_(y2)N (x1>x2 andx1+y1=x2+y2=1) multiple quantum wells or superlattice structure. Thep-region could include/comprise an electron blocking layer (single ormultiple quantum barrier layers) to prevent the overflow of minoritycarriers (electrons) into the p-region.

It is thus a preferred embodiment if the nanowire or nanopyramid isprovided with a multiple quantum well. It is thus a preferred embodimentif the nanowire or nanopyramid is provided with an electron blockinglayer. Ideally, the nanowire or nanopyramid is provided with both anelectron blocking layer and a multiple quantum well.

It is thus an embodiment of the invention to employ a multistep, such astwo step growth procedure, e.g. to separately optimize the nanowire ornanopyramid nucleation and nanowire or nanopyramid growth.

A significant benefit of MBE is that the growing nanowire or nanopyramidcan be analysed in situ, for instance by using reflection high-energyelectron diffraction (RHEED). RHEED is a technique typically used tocharacterize the surface of crystalline materials. This technologycannot be applied so readily where nanowires or nanopyramids are formedby other techniques such as MOVPE.

A significant benefit of MOVPE is that the nanowires or nanopyramids canbe grown at a much faster growth rate. This method favours the growth ofradial heterostructure nanowires or nanopyramids and microwires, forexample: n-type doped GaN core with shell consisting of intrinsicAlN/Al(In)GaN multiple quantum wells (MQW), AlGaN electron blockinglayer (EBL), and p-type doped (Al)GaN shell. This method also allows thegrowth of axial heterostructured nanowire or nanopyramid usingtechniques such as pulsed growth technique or continuous growth modewith modified growth parameters for e.g., lower VIII molar ratio andhigher substrate temperature.

In more detail, the reactor must be evacuated after placing the sample,and is purged with N₂ to remove oxygen and water in the reactor. This isto avoid any damage to the graphene at the growth temperatures, and toavoid unwanted reactions of oxygen and water with the precursors. Thetotal pressure is set to be between 50 and 400 Torr. After purging thereactor with N₂, the substrate is thermally cleaned under H₂ atmosphereat a substrate temperature of about 1200° C. The substrate temperaturecan then be set to a temperature suitable for the growth of the nanowireor nanopyramid in question. The growth temperature may be in the range700 to 1200° C. The temperature employed is, however, specific to thenature of the material in the nanowire or nanopyramid. For GaN, apreferred temperature is 800 to 1150° C., e.g. 900 to 1100° C., such as1100° C. For AlGaN the range is slightly higher, for example 900 to1250° C., such as 1050 to 1250° C., e.g. 1250° C.

The metal organic precursors can be either trimethylgallium (TMGa), ortriethylgallium (TEGa) for Ga, trimethylalumnium (TMAl) ortriethylalumnium (TEAl) for Al, and trimethylindium (TMIn) ortriethylindium (TEIn) for In. The precursors for dopants can be SiH₄ forsilicon and bis(cyclopentadienyl)magnesium (Cp₂Mg) orbis(methylcyclopentadienyl)magnesium ((MeCp)₂Mg) for Mg. The flow rateof TMGa, TMAl and TMIn can be maintained between 5 and 100 sccm. The NH₃flow rate can be varied between 5 and 150 sccm.

In particular, the simple use of vapour-solid growth may enable nanowireor nanopyramid growth. Thus, in the context of MBE, simple applicationof the reactants, e.g. In and N, to the substrate without any catalystcan result in the formation of a nanowire or nanopyramid. This forms afurther aspect of the invention which therefore provides the directgrowth of a semiconductor nanowire or nanopyramid formed from theelements described above on a graphitic substrate. The term directimplies therefore the absence of a catalyst to enable growth.

Viewed from another aspect the invention provides a composition ofmatter comprising a plurality of group III-V nanowires or nanopyramidsgrown epitaxially on a graphitic substrate, preferably through the holesof a hole-patterned mask on said graphitic substrate, said nanowires ornanopyramids comprising:

an n-type doped region and a p-type doped region separated by anintrinsic region which acts as a multiple quantum well, said p-typedoped region comprising an electron blocking layer.

Said regions can be represented by layers within a nanowire ornanopyramid or shells on a core to create the nanowire or nanopyramid.Thus, the invention further provides a plurality of radial group III-Vnanowires or nanopyramids grown epitaxially on a graphitic substratecomprising, in this order, an n-type doped core with shell comprising anintrinsic multiple quantum well, an electron blocking shell (EBL), andp-type doped shell. The n-type doped region could include/comprise ahole blocking layer (single or multiple quantum barrier layers) toprevent the overflow of minority charge carriers (holes) into the n-typedoped region.

Top Contact

In order to create a device of the invention, the top of the nanowiresor nanopyramids needs to comprise a top electrode and, for the LEDembodiment preferably a reflective layer. In some embodiments, theselayers can be one in the same.

In one preferred embodiment, a top contact is formed using anothergraphitic layer. The invention then involves placing a graphitic layeron top of the formed nanowires or nanopyramids to make a top contact. Itis preferred that the graphitic top contact layer is substantiallyparallel with the substrate layer. It will also be appreciated that thearea of the graphitic layer does not need to be the same as the area ofthe substrate. It may be that a number of graphitic layers are requiredto form a top contact with a substrate with an array of nanowires ornanopyramids.

The graphitic layers used can be the same as those described in detailabove in connection with the substrate. The top contact is graphitic,more especially it is graphene. This graphene top contact should containno more than 10 layers of graphene or its derivatives, preferably nomore than 5 layers (which is called as a few-layered graphene).Especially preferably, it is a one-atom-thick planar sheet of graphene.

The crystalline or “flake” form of graphite consists of many graphenesheets stacked together (i.e. more than 10 sheets). It is preferred ifthe top contact is 20 nm in thickness or less. Even more preferably, thegraphitic top contact may be 5 nm or less in thickness.

When graphene contacts directly to the semiconductor nanowires ornanopyramids, it usually forms a Schottky contact which hinders theelectrical current flow by creating a barrier at the contact junction.Due to this problem, the research on graphene deposited onsemiconductors has been mainly confined to the use ofgraphene/semiconductor Schottky junctions.

Application of the top contact to the formed nanowires or nanopyramidscan be achieved by any convenient method. Methods akin to thosementioned previously for transferring graphitic layers to substratecarriers may be used. The graphitic layers from Kish graphite, highlyordered pyrolytic graphite (HOPG), or CVD may be exfoliated bymechanical or chemical methods. Then they can be transferred intoetching solutions such as HF or acid solutions to remove Cu (Ni, Pt,etc.) (especially for CVD grown graphitic layers) and any contaminantsfrom the exfoliation process. The etching solution can be furtherexchanged into other solutions such as deionised water to clean thegraphitic layers. The graphitic layers can then be easily transferredonto the formed nanowires or nanopyramids as the top contact. Againe-beam resist or photoresist may be used to support the thin graphiticlayers during the exfoliation and transfer processes, which can beremoved easily after deposition.

It is preferred if the graphitic layers are dried completely afteretching and rinsing, before they are transferred to the top of thenanowire or nanopyramid arrays. To enhance the contact between graphiticlayers and nanowires or nanopyramids a mild pressure and heat can beapplied during this “dry” transfer.

Alternatively, the graphitic layers can be transferred on top of thenanowire or nanopyramid arrays, together with a solution (e.g. deionisedwater). As the solution dries off, the graphitic layers naturally form aclose contact to underlying nanowires or nanopyramids. In this “wet”transfer method, the surface tension of the solution during the dryingprocess might bend or knock out the nanowire or nanopyramid arrays. Toprevent this, where this wet method is used, more robust nanowires ornanopyramids are preferably employed. Nanowires or nanopyramids having adiameter of >80 nm might be suitable. Alternatively, hole patternedsubstrates which support the vertical nanowire or nanopyramid structurecould be used. One may also use the critical-point drying technique toavoid any damage caused by surface tension during the drying process.Another way to prevent this is to use supporting and electricallyisolating material as fill-in material between nanowires ornanopyramids. The fill-in material needs to be transparent to theemitted light. We discuss the use of fillers below.

If there is a water droplet on a nanowire or nanopyramid array andattempts to remove it involve, for example a nitrogen blow, the waterdrop will become smaller by evaporation, but the drop will always try tokeep a spherical form due to surface tension. This could damage ordisrupt the nanostructures around or inside the water droplet.

Critical point drying circumvents this problem. By increasingtemperature and pressure, the phase boundary between liquid and gas canbe removed and the water can be removed easily.

Also doping of the graphitic top contact can be utilized. The majorcarrier of the graphitic top contact can be controlled as either holesor electrons by doping. It is preferable to have the same doping type inthe graphitic top contact and in the semiconducting nanowires ornanopyramids.

It will be appreciated therefore that both top graphitic layer and thesubstrate can be doped. In some embodiments, the substrate and/or thegraphitic layer is doped by a chemical method which involves with anadsorption of organic or inorganic molecules such as metal chlorides(FeCl₃, AuCl₃ or GaCl₃), NO₂, HNO₃, aromatic molecules or chemicalsolutions such as ammonia.

The surface of substrate and/or the graphitic layer could also be dopedby a substitutional doping method during its growth with incorporationof dopants such as B, N, S, or Si.

Reflective Layer/Electrode

The device is provided with two electrodes. A first electrode is placedin contact with the graphene substrate. That electrode might be based ona metal element such as Ni, Au, Ti, or Al or a mixture thereof or astack thereof, such as a stack Ti/Al/Ni/Au. Pd, Cu or Ag might also beused. Often the first electrode will be the n electrode. The electrodemay be on either surface of the graphitic substrate, preferably on thesame surface as the grown nanowires or nanopyramids.

A second electrode is placed as a top contact on top of the grownnanowires or nanopyramids. This electrode will often be the p electrode.It is preferred if this forms a good ohmic contact with the nanowires ornanopyramids. Suitable electrode materials include Ni, Ag, Pd and Cu. Inparticular, a Ni/Au stack could be used. This electrode might also actas a heat sink. As discussed below in further detail, the LED device ofthe invention is preferably in the form of a flip chip. The top contactelectrode therefore sits at the bottom of the flip chip assembly. It istherefore preferred if the electrode either reflects light or isprovided with a light reflective layer. The light reflective layer isideally metallic. The light-reflective contact layer can be formed inseveral ways, although using a PVD (Physical Vapour Deposition) methodand well-known mask techniques is the preferred method. The reflector ispreferably made of aluminum or silver, but other metals or metal alloysmay also be used. The purpose of the light-reflective layer is toprevent light from leaving the structure in a direction other than thepreferred direction, and to focus the emitted light to one singledirection. Additionally, the light-reflective layer may function as atop contact electrode to the nanowires or nanopyramids. The lightemitted by the LED is channeled in a direction opposite to thereflective layer, i.e. out the top of the flip-chip. Where a graphenetop contact layer is present, a light reflective layer is preferablyadditionally present.

The reflective layer needs to reflect light and may also act as a heatsink. Suitable thickness are 20 to 400 nm, such as 50 to 200 nm.

In the photodetector embodiment, there is no need to use a reflectivelayer but such a layer could be used, perhaps to reflect incoming lightonto the nanowires or nanopyramids to enhance photodetection.

Filler

It is within the scope of the invention to use a filler to surround theflip chip assembly as long as the filler is transparent, e.g. to UVlight. Filler may be present in the space between nanowires ornanopyramids and/or around the assembly as a whole. Different fillersmight be used in the spaces between the nanowires or nanopyramids thanin the assembly as a whole.

Applications

The invention relates to LEDs, in particular UV LEDs and especiallyUV-A, UV-B, or UV-C LEDs. The LEDs are preferred designed as a so called“flip chip” where the chip is inverted compared to a normal device.

The whole LED arrangement can be provided with contact pads forflip-chip bonding distributed and separated to reduce the average seriesresistance. Such a nanostructured LED can be placed on a carrier havingcontact pads corresponding to the position of p-contact pads andn-contact pads on the nanowire or nanopyramid LED chip and attachedusing soldering, ultrasonic welding, bonding or by the use ofelectrically conductive glue. The contact pads on the carrier can beelectrically connected to the appropriate power supply lead of the LEDpackage.

Nanowire-based LED devices as such, are usually mounted on a carrierthat provides mechanical support and electrical connections. Onepreferred way to construct a LED with improved efficiency is to make aflip-chip device. A light reflective layer with high reflectivity isformed on top of the nanowires or nanopyramids. The initial support canbe removed as a part of the process, leaving the substrate layer, toallow for the light to be emitted through said substrate layer which hasformed a base for the nanowires or nanopyramids. If the support istransparent then of course there is no need to remove it. Emitted lightdirected towards the top of the nanowires or nanopyramids is reflectedwhen it encounters the reflective layer, thus creating a clearlydominating direction for the light leaving the structure. This way ofproducing the structure allows for a much larger fraction of the emittedlight to be guided in a desired direction, increasing the efficiency ofthe LED. The invention therefore enables the preparation of visible LEDsand UV LEDs.

The invention also relates to photodetectors in which the device absorbslight and generates a photocurrent. The light reflective layer mayreflect light entering the device back on to the nanowires ornanopyramids for enhanced light detection.

The invention will now be further discussed in relation to the followingnon limiting examples and figures.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 shows a possible flip chip design. In use therefore, light isemitted through the top of the device (marked hυ). Support 1 ispreferably formed from fused silica (best option), quartz, siliconcarbide, sapphire or AlN. The use of other transparent supports is alsopossible. The use of fused silica or quartz is preferred. In use, thesupport, if still present, is positioned upper most in the device andhence it is important that the support is transparent to the emittedlight and thus allows light out of the device.

Layer 2, which is a preferred optional layer, is positioned between thesupport and the graphene layer 3 in order to reduce the sheet resistanceof graphene. Suitable materials for layer 2 include inert nitrides suchas hBN or a metallic nanowire network such as Ag nanowire network ormetal grid.

Layer 3 is the graphene layer which can be one atomic layer thick or athicker graphene layer, such as one which is up to 20 nm in thickness.

Nanowires 4 are grown from substrate layer 3 epitaxially. Ideally, thenanowires are formed from Al(In)GaN, AlN or GaN and are doped to createn-i-p or n-p junctions.

A filler 5 can be positioned between grown nanowires. A topelectrode/light reflective layer 6 is positioned on top of nanowires 4.The light reflective layer may also be provided with a p-electrodecomprising Ni or Au. In use, this layer reflects any light emitted bythe device to ensure that the light is emitted through the top of thedevice opposite the reflective layer. This is the so called flip chiparrangement as the device is upside down compared to a conventional LED.

Electrode 10 is positioned on the graphene layer 3. That electrode mightcomprise Ti, Al, Ni or/and Au. The graphene layer may be provided with amask 7 to allow growth of the nanowires in definitive positions on thegraphene.

The whole device is soldered to conductive tracks/pads 13 on a submount8 via solder layer 9.

When a forward current is passed across the device, visible or UV light,dependent on composition of matter, is generated in the nanowires and isemitted, possibly after reflecting off the reflective layer out the topof the device.

When a reverse current is passed across the device and when the deviceis exposed to visible or UV light, the nanowires absorb the visible orUV light, dependent on composition of matter, and converts it intocurrent, working as a photodetector.

FIG. 2 shows a potential nanowire of the invention. The nanowire isprovided with different components in an axial direction by variation ofthe elements being supplied during the growing phase. Initially, ann-type doped GaN material is deposited, followed by n-AlN or n-(Al)GaN.In the central section of the nanowire as shown are a series of multiplequantum wells formed from (In)(Al)GaN. There follows the p-doped regionbased on AlGaN or (Al)GaN, and an electron blocking layer based onp-Al(Ga)N and finally a p-GaN layer.

FIG. 3 shows an alternative chip design in which the nanowires are grownradially creating core shell structures. In use therefore, light isemitted through the top of the device (marked hυ). Support 1 ispreferably formed from fused silica or quartz. In use, the support, ifstill present, is positioned upper most in the device and hence it isimportant that the support is transparent to the emitted light and thusallows light out of the device.

Layer 2, which is a preferred intermediate layer, is positioned betweenthe support and the graphene layer 3 in order to reduce the sheetresistance of graphene. Suitable materials for layer 2 include inertnitrides such as hBN or a metallic nanowire network such as silvernanowire network or metal grid.

Layer 3 is the graphene layer which can be one atomic layer thick or athicker graphene layer, such as one which is up to 20 nm in thickness.

Nanowires 4 are grown from layer 3 epitaxially. Ideally, the nanowiresare formed from Al(In)GaN, AlN or GaN and are doped to create n-i-p orn-p junctions. The graphene can be provided with a mask layer 7.

A filler 5 can be positioned between grown nanowires. A topelectrode/light reflective layer 6 is positioned on top of nanowires 4.The light reflective layer may also be provided with a p-electrodecomprising Ni or/and Au or may itself be an electrode. In use, thislayer reflects any light emitted by the device to ensure that the lightis emitted through the top of the device opposite the reflective layer.This is the so called flip chip arrangement as the device is upside downcompared to a conventional LED.

Electrode 10 is positioned on the graphene layer 3. When a forwardcurrent is passed across the device, visible or UV light, dependent oncomposition of matter, is generated in the nanowires and is emitted,possibly after reflecting off the reflective layer out the top of thedevice.

The whole device is soldered to conductive tracks/pads 13 on a submount8 via solder layer 9.

When a reverse current is passed across the device and when the deviceis exposed to visible or UV light, the nanowires absorb the visible orUV light, dependent on composition of matter, and converts it intocurrent, working as a photodetector.

FIG. 4 shows a nanowire grown radially but having the same components asthose of FIG. 2 in a shell arrangement. The nanowire is provided withdifferent components in a radial direction by variation of the elementsbeing supplied during the growing phase. Initially, an n-doped GaNmaterial is deposited, followed by n-AlN or n-(Al)GaN. In the centralshell of the nanowire as shown are a series of multiple quantum wellsformed from (In)(Al)GaN. There follows the p-doped region based onAl(Ga)N, and an electron blocking shell based on p-Al(Ga)N and finally ap-GaN shell.

FIG. 5 shows a photodetector. In use therefore, light is acceptedthrough the top of the device. Support 1 is preferably formed from fusedsilica, quartz, silicon carbide or AlN. The use of fused silica orquartz is preferred. In use, the support, if still present, ispositioned upper most in the device and hence it is important that thesupport is transparent to the accepted light and thus allows light in tothe device.

Layer 2, which is a preferred optional layer, is positioned between thesupport and the graphene layer 3 in order to reduce the sheet resistanceof graphene. Suitable materials for layer 2 include inert nitrides suchas hBN or a metallic nanowire network such as Ag nanowire network ormetal grid.

Layer 3 is the graphene layer which can be one atomic layer thick or athicker graphene layer, such as one which is up to 20 nm in thickness.

Nanowires 4 are grown from substrate layer 3 epitaxially. Ideally, thenanowires are formed from Al(In)GaN, AlN or GaN and are doped to createn-i-p or n-p junctions.

A filler 5 can be positioned between grown nanowires. A top electrodelayer 11 is positioned on top of nanowires 4. This electrode is ideallya p-electrode comprising Ni or Au.

Electrode 10 is positioned on the graphene layer 3. The graphene layermay be provided with a mask 7 to allow growth of the nanowires indefinitive positions on the graphene.

The whole device is soldered to conductive tracks/pads 13 on a submount8 via solder layer 9.

When a reverse current is passed across the device and when the deviceis exposed to visible or UV light, the nanowires absorb the visible orUV light, dependent on composition of matter, and converts it intocurrent, working as a photodetector.

FIG. 6: (a) Schematic diagram showing the growth of nanowires ongraphite flake and the top and bottom contacts to the nanowires. Tiltedview SEM image (b) and high-resolution SEM image (c) of selectivelygrown GaN nanowires on multi-layer graphene flakes by MBE.

The graphite flake 3 (or graphene) is transferred on a support substratesuch as fused silica substrate 1. A mask material 7 such as Al₂O₃ andSiO₂ is deposited on the graphite flake. A big hole of 10 μm in diameteris etched in the mask material using photolithography such that thegraphite surface is exposed in the hole. The sample is transferred intothe MBE chamber for the nanowire growth. The substrate is heated to thegrowth temperature and a nucleation layer consisting of Al and AlN isdeposited on the substrate, which is followed by the initiation of the(Al)GaN nanowires/nanopyramids growth.

FIG. 7: Tilted view SEM images of GaN nanowires grown on (a) multi-layergraphene flakes by MBE. (b) GaN nanowires grown on hole patternedmulti-layer graphene flakes by MBE.

The graphite flake (or graphene) is transferred on a support substratesuch as fused silica substrate. A mask material such as Al₂O₃ and SiO₂is deposited on the graphite flake. A big hole of 1 μm in diameter andseveral small holes of ˜80 nm in diameter is etched in the mask materialusing e-beam lithography such that the graphite surface is exposed inthe holes. The sample is transferred into the MBE chamber for nano wiregrowth. The substrate is heated to the growth temperature and anucleation layer consisting of Al and AlN is deposited on the substrate,which is followed by the initiation of the (Al)GaN nanowires are grown.FIG. 7 shows the tilted view SEM image of GaN nanowires grown in the bighole region (a) and small hole patterns (b).

FIGS. 8a and b shows the growth of nanopyramids. Support 1 is preferablyformed from fused silica (best option), quartz, silicon carbide,sapphire or AlN. The use of other transparent supports is also possible.The use of fused silica or quartz is preferred. In use, the support, ifstill present, is positioned upper most in the device and hence it isimportant that the support is transparent to the emitted light and thusallows light out of the device.

Layer 3 is the graphene layer which can be one atomic layer thick or athicker graphene layer, such as one which is up to 20 nm in thickness.

Nanopyramids 40 are grown from layer 3 epitaxially. Ideally, thenanopyramids are formed from Al(In)GaN, AlN or GaN and are doped tocreate n-i-p or n-p junctions. A core shell nanopyramid can be grown bechanging the nature of the flux supplied during the growth period.

A filler 5 (not shown) can be positioned between grown nanopyramids. Atop electrode/light reflective layer (not shown) can be positioned ontop of nanopyramids. The light reflective layer may also be providedwith a p-electrode comprising conducting materials such as Ni or Au. Inuse, this layer reflects any light emitted by the device to ensure thatthe light is emitted through the top of the device opposite thereflective layer. This is the so called flip chip arrangement as thedevice is upside down compared to a conventional LED.

The graphene layer may be provided with a mask 7 to allow growth of thenanopyramids in definitive positions on the graphene.

FIG. 9: (a) Low magnification and (b) High magnification tilted view SEMimages of GaN nanopyramids grown on patterned single or double layergraphene by MOVPE.

A graphene layer is transferred on a support substrate such as fusedsilica substrate. A mask material such as Al₂O₃ and SiO₂ is deposited onthe graphene. Several small holes of ˜100 nm in diameter and pitch inthe range between 0.5 and 5 μm are etched in the mask material usinge-beam lithography such that the graphene surface is exposed in theholes. The sample is then transferred into the MOVPE reactor for thenanopyramid growth. The substrate is heated to the growth temperatureand a nucleation layer consisting of AlGaN is deposited on thesubstrate, which is followed by the growth of the (Al)GaN nanopyramids.FIG. 9 shows the tilted view SEM image of patterned GaN nanopyramidsgrown on graphene.

1. A device comprising: a plurality of nanowires or nanopyramids grown on a graphitic substrate, the plurality of nanowires or nanopyramids having a p-n or p-i-n junction and each of the plurality of nanowires having a top and a bottom, the bottom of each of the plurality of nanowires or nanopyramids being in contact with the graphitic substrate, a first electrode in electrical contact with the graphitic substrate; a second electrode in contact with the top of at least a portion of the plurality of nanowires or nanopyramids; wherein the plurality of nanowires or nanopyramids comprise at least one group III-V compound semiconductor.
 2. The device of claim 1, further comprising: a light reflective layer, wherein the light reflective layer is: in contact with the top of at least a portion of the plurality of nanowires or nanopyramids or wherein the light reflective layer is in contact with the second electrode; and wherein the device comprises a light emitting diode that emits light in a direction substantially opposite to the light reflective layer.
 3. The device of claim 1, wherein the device comprises a photodetector device and wherein in use light is absorbed in the device.
 4. The device of claim 1, wherein the device further comprises a hole-patterned mask deposited on the graphitic substrate, the hole-patterned mask comprising a plurality of holes, and wherein the plurality of nanowires or nanopyramids are grown through the holes of the hole-patterned mask on the graphitic substrate.
 5. The device of claim 1, wherein the plurality of nanowires or nanopyramids are grown epitaxially on the graphitic substrate.
 6. The device of claim 1, wherein the graphitic substrate is graphene.
 7. The device of claim 1, wherein the graphitic substrate has a thickness of 20 nm or less.
 8. The device of claim 1, wherein the graphitic substrate is graphene having up to 10 atomic layers.
 9. The device of claim 1, wherein the device further comprises a support adjacent the graphitic substrate, opposite to the plurality of nanowires or nanopyramids grown on the graphitic substrate.
 10. The device of claim 9, wherein the support is fused silica or quartz.
 11. The device of claim 1, wherein the device further comprises an intermediate layer adjacent the graphitic substrate, opposite to the plurality of nanowires or nanopyramids grown on the graphitic substrate.
 12. The device of claim 11, wherein the intermediate layer is hexagonal boron nitride (hBN), a metal grid, or a Ag nanowire network.
 13. The device of claim 1, wherein the plurality of nanowires or nanopyramids comprise GaN, AlGaN, InGaN, or AlInGaN.
 14. The device of claim 1, wherein the plurality of nanowires or nanopyramids comprise a multiple quantum well.
 15. The device of claim 1, wherein the plurality of nanowires or nanopyramids contain an electron blocking layer.
 16. The device of claim 1, wherein the device emits or absorbs light in the UV spectrum.
 17. The device of claim 1, wherein the device comprises a plurality of nanowires and the p-n or p-i-n junction within each of the plurality of nanowires is axial.
 18. The device of claim 1, wherein the plurality of nanowires or nanopyramids comprise a tunnel junction.
 19. The device of claim 1, wherein the plurality of nanowires or nanopyramids comprise an (Al)GaN/Al(Ga)N superlattice.
 20. The device of claim 1, wherein the plurality of nanowires or nanopyramids comprise AlGaN with an increasing or decreasing concentration of Al along a direction in each of the plurality of nanowires or nanopyramids.
 21. The device of claim 1, wherein the plurality of nanowires or nanopyramids are doped using Mg or Be.
 22. The device of claim 1, wherein the device comprises a light emitting diode device, each of the plurality of nanowires or nanopyramids is separated by space, and the space between each of the plurality of nanowires or nanopyramids is filled by a supporting and electrically isolating filler material transparent to the light emitted from the device.
 23. The device of claim 3, wherein each of the plurality of nanowires or nanopyramids is separated by space and the space between each of the plurality of nanowires or nanopyramids is filled by a supporting and electrically isolating filler material transparent to visible and/or UV light entering into the device.
 24. A composition of matter comprising: a plurality of group III-V nanowires or nanopyramids grown epitaxially on a graphitic substrate, wherein the plurality of group III-V nanowires or nanopyramids comprise an n-type doped region and a p-type doped region separated by an intrinsic region which acts as a multiple quantum well, wherein the p-type doped region comprises an electron blocking layer. 